Degradation of Acetophenone in Water by Pulsed Corona Discharges

Plasma Chemistry and Plasma Processing, Vol. 20, No. 3, 2000

Degradation of Acetophenone in Water by Pulsed Corona Discharges Y. Wen and X. Jiang1 Received March 24, 1999; revised December 7, 1999

Degradation of acetophenone in dilute aqueous solution has been studied using pulsed corona discharges in water. Higher conversions of acetophenone were obtained with the addition of oxygen or ozone than with the addition of nitrogen and without the addition of any gas. Intermediates of acetophenone degradation, as determined by gas chromatography–mass spectroscopies (GC–MS), were phenethyl alcohol, toluene, and 2-acetylphenol. In addition, the degradation reaction pathways of acetophenone in water are discussed. KEY WORDS: Degradation; acetophenone in water; pulsed corona discharges; gas effect; reaction pathway.

1. INTRODUCTION In the past few years, interest in the development of advanced oxidation technologies (AOTs) for the treatment of hazardous chemical water has grown enormously, for example, photocatalysis, ultrasonic irradiation, electron-beam irradiation, and electrohydraulic discharge process.(1–3) Despite advances in the above techniques, there still remains a need to treat the vast quantities of organic waste released into the environment from a wide variety of processes. Pulsed corona discharge is now considered a promising tool for the cleaning of polluted gas flows. The degradations of the volatile organic compounds (VOCs) using corona discharges in air have been extensively investigated.(4–6) In the gas phase, pulsed corona discharge was found to be much more effective at promoting the reactions leading to the degradation of volatile organic compounds than, for example, electron beam processes.(7) However, less research has been performed on the degradation of organic compounds in water using pulsed corona discharges.(7–9) Discharges in water differ from those in gas phase since water is a polar liquid with high conductivity (dielectric constant ε G80). The discharges and chemical reactions in 1

Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang, P.R. China. 343 0272-4324y00y0900-0343$18.00y0  2000 Plenum Publishing Corporation

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water are fairly complicated. However, there are similarities, such as the formation of streamers, which occur as precursors to breakdown water. The corona discharge in water is a nonthermal treatment technology that injects energy into an aqueous solution through a plasma channel formed by a high-voltage electrical discharge between two submersed electrodes.(5) In this case, the formation of prebreakdown streamers quickly leads to discharge, especially when a streamer filament forms a bridge between the electrodes. Clements et al.(8) investigated prebreakdown phenomena in water for point-plate geometries using high-voltage pulse. Their study showed that pulsed corona discharges in aqueous solutions with oxygen bubbling into the reactor produced large amounts of ozone that can, in turn, lead to the decolorization of dyes. The discharge characteristics were found to be extremely sensitive to changes in the solution conductivity and the polarity and magnitude of the applied voltage. However, they did not propose a mechanism of dye breakdown. Lately, Sharma(7) reported that pulsed corona discharges in water are effective at breaking down phenol in aqueous solutions. Two simultaneous degradation pathways of phenol were proposed. The first pathway consisted of corona-induced aqueous phase reactions; the second pathway arose from ozone production in the gas phase with subsequent mass transfer into the liquid phase followed by liquid-phase ozone reactions. To design and operate reactors for waste treatment, it is crucial to explore reaction intermediates and develop a fundamental understanding of the chemical reaction pathways involved in the breakdown of organic compounds. Acetophenone (hereafter referred to as AP) represents one class of the organic compounds, aromatic ketones, which has been found in air, soil, and various waters.(10) However, little is known of its oxidation in aqueous medium by corona discharges in water. In this paper, we explore the effects of gas (oxygen, nitrogen and ozone) on the acetophenone degradation. The primary objective of this study was to understand chemical reaction pathways of acetophenone degradation in water induced by pulsed corona discharges. 2. EXPERIMENTAL PROCEDURE The acetophenone used in this study was analytical reagent grade. Nitrogen and oxygen gases were high purity. Oxygen or nitrogen from gas cylinder was passed through silica gel, activated carbon, and molecular sieve columns for purification. Ozone was generated with an ozonator with a flow of 200 mLymin. The O3 gas phase concentration was 14.4 mgyL. Doubly distilled water was used throughout this study.

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Fig. 1. The schematic diagram of the experimental apparatus R, 1 MΩ; L, 1 µH; C, 0.1 µF, power 60 W; a, needle electrode; reactor, 21.7 mm I.D. and 260 mm length.

Upon degradation of AP in water by pulsed corona discharges, a laboratory prototype of the treatment system was designed and constructed. As presented in Fig. 1, the degradation system consists of two major components: a high-voltage pulse generator and a corona reactor. The reactor is a cylindrical Pyrex tube (21.7 mm I.D. and 260 mm length). A stainless steel hypodermic needle (common 7 injection needle) was inserted into the reactor from the bottom, through silicone sealing insulation. The needle was connected with the high-voltage (HV) dc source through a rotary spark gap switch. The testing gases (O2 , N2 , or O3 ) were bubbled through the hypodermic needle with a flow of 200 mLymin. A stainless steel disk of 1.8 cm diameter, attached to an earthen stainless steel rod in the shape of a piston, was suspended from the top of silicone sealing. The point-to-plate distance is 2 cm, except for the experiment of direct pulsed corona discharges in water, in which the point-to-plate distance is 1 mm. The pulsed power supply consists of a HV dc source (30 kV), resister R (1 MΩ), inductance L (1 µH), rotating spark gap, and a HV capacitor C (0.1 µF) (as shown in Fig. 1). The capacitor C was charged through resister R and inductance L by the high-voltage dc power supply. When the rotating spark gap reached the point of conduction, the electrical energy stored in capacitance C is discharged through the rotating spark gap switch and reactor, generating a high-voltage pulse of 8-µs width, 1-µs rise time. The voltage and pulse frequency of the source were 30 kV, 50–150 Hz. The input power is 60 W. Positive polarity was used in this study. The output voltages were measured using an oscilloscope (XJ4362A), along with a 310 :1 high-voltage divider. The gas effect on the degradation of AP was investigated at room temperature. Oxygen, nitrogen, and ozone were used individually. The conversion of AP was compared to determine the importance of gas. In the case

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of ozonation experiments, the dc source was off. The ozone was bubbled, for various intervals of time, through water sample containing AP. The combination of ozone and pulsed corona discharges in water on degradation of AP was also studied. The volume of aqueous solution AP was 100 cm3. The concentration of AP was 250 mgyL. Samples were analyzed by GC (1102G), equipped with an OV-101 glass capillary column (30 m length), and flame ionization detector. The approximate concentration of AP could be calculated from the peak area. The conversion was calculated using the following equation: AP conversion %G

C0AC1 B% C0

(1)

where C0 Gthe initial concentration of AP and C1 Gthe instant concentration of AP. Ozone (O3 ) concentration in the gas phase was analyzed by standard analyical procedure.(11) The sample gas was passed through an absorbent solution (2% KI and 0.4% NaOH in deionized water, used ∼4 times in excess than required) for a known interval (5–10 min). Then, adding 5 M H2SO4 solution, the liberated iodine was titrated against standardized Na2S2O3 solution (0.005 molyL). Ozone concentration as calculated uses the following formula: Ozone (molyL)G(AJB)MyV

(2) 3

where A is the volume of Na2S2O3 solution used (cm ), B is the volume of Na2S2O3 solution used during blank run (cm3 ), M is the concentration of Na2S2O3 (molyL), and V is the gas sample volume (cm3 ). Intermediates in the degradation reactions were analyzed by the following procedure. The sample of treated solution was first extracted for 2 hr using 10 ml dichloromethane. The organic extract was then concentrated to small volume by flowing dry N2 . Subsequently, the sample was analyzed using a HP6890y5973 GC–MS with HP-5MS capillary column of 30 m length. The intermediates were determined by matching with the standard samples, considering both elution time and mass spectrum. 3. RESULTS AND DISCUSSION In pulsed corona discharges, electrons gain sufficient energy from the electric field for collisional ionization, the resulting exponential increase in current (electron avalanche) leading to discharge. The current in the highfield region causes local heating, which vaporizes the liquid, forming bubbles. Gas breakdown occurs within each bubble, causing further heating, and the bubbles grow until the complete breakdown of the gap occurs,


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which results in the formation of plasma channels and chemical degradations. When positive dc high-voltage of 30 kV was applied to the needle electrode, the average current was 2 mA. The discharges in water took place and the purple color of light emission was observed. A comparison of conversions of AP after pulsed corona discharge is given in Fig. 2. While the initial concentrations remain constant (250 mgyL), the conversions of AP increase with discharge time as shown in Fig. 2a. Approximately 45% conversion was achieved after 60 min when no gas was added to the system. Because of the high temperatures of plasma channel, the efficiency for degrading AP in plasma channel is quite high. Even with short residence times, AP is degraded in milliseconds, and the intermediate products were able to form. On the other hand, the UV emitted from the plasma channel is absorbed immediately by the water layer surrounding the plasma channel, resulting in photolysis and collisional dissociation. Due to high density of water, the electric field strength required for discharges in water is high. When a positive dc high-voltage of 30 kV was applied, the electrode distance

Fig. 2. Conversion of acetophenone as a function of discharge time. (a) No gas; (b) addition of gases (flow rate 200 mLymin).

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for discharges in water is only 1 mm in our experiments. The small volume of plasma channel limited the amount of AP that can be degraded due to pyrolysis, photolysis, and collisional dissociation. Therefore, the conversion of AP is fairly low. Comparing Fig. 2b with 2a, it can be observed that, on the addition of gases (oxygen or ozone), there are obvious increases in conversion of AP, which are up to 75–92% after 30 min. These results show that the addition of gas clearly enhances the degradation efficiency. The enhancement of gaseous electric field due to high dielectric constant (s G80) of water can easily result in gas discharges and formation of high-temperature (>1400 K) plasma channels.(9) Furthermore, the addition of oxygen or ozone to the reactor remarkably increases the maximum distance between electrodes where discharges may occur. Therefore active plasma zones greatly increase, which leads to more pyrolysis, photolysis, and collisional dissociation. In addition, when gases were bubbling into solution, the following electron– molecule reactions were taken into account: eCO2 →eCO2 (α 1∆)

(3)

eCO2 → eC·OC·O

(4)

eCO2 → eC·OC·O( D) 1

(5)

eCH2O→eC·OHC·H

(6)

eCO3 → eC·OCO2

(7)

·OCH2O→2·OH

(8)

·OHCO3 →HO CO2

(9)

A 2

The major free radicals are hydroxyl radicals that are highly reactive with a broad range of organic compounds and are generally considered crucial for the degradation of most organic waste contaminant. Thus, in addition to oxygen or ozone, pulsed corona discharges in water can generate many more hydroxyl radicals than without addition of oxygen or ozone. Thus, the conversion of AP increases. Similar sets of experiments were run using N2 instead of oxygen. The variation of AP conversions is presented in Fig. 2b. It can be observed that higher conversions of AP are achieved with oxygen than with nitrogen. As reported above, high-energy electrons are typically considered to react primarily with molecular oxygen, resulting in efficient production of hydroxyl radicals. Furthermore, Kulikovsky(12) reported that the dissociation of oxygen molecules requires less energy than the dissociation of nitrogen molecules. The dissociation of nitrogen molecules occurs mainly around the head of the streamer, while the dissociation of oxygen molecules also occurs


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at the low-electric field strengthynumber density of molecules (EyN) values around plasma channel, and the largest parts of the species produced by pulsed corona discharges are O atoms and excited nitrogen molecules. Thus, in the case of nitrogen feed, less collisional dissociation was obtained. An important observation from Fig. 2b is that AP conversion can be ranked: (ozoneCpulsed corona discharges in water)H(oxygenCpulsed corona discharges in water)H(nitrogenCpulsed corona discharges in water)H ozone. High-energy electrons in pulsed corona discharges can dissociate ozone to oxygen and the O atom, which reacts almost instantaneously with water in its neighborhood, results in the production of hydroxyl radicals. Because of the enhancement in ozone decomposition by the pulsed corona discharges in water, the rate of ozone transfer into water should increase. Therefore, AP conversion by combination of ozonation and pulsed corona discharges in water increases. The intermediates of AP degradation, as determined by GC–MS analysis, are as follows: toluene, phenethyl alcohol, and 2-acetylphenol (as shown

Fig. 3. The spectrum of intermediates determined by GC–MS upon addition of oxygen. (a) phenylethyl alcohol; (b) toluene; (c) 2-acetylphenol.

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in Fig. 3). A plausible degradation mechanism for AP, as described in literature,(8,9) can explain the formation of toluene and 2-acetylphenol resulting from pyrolysis, photolysis, and rearrangement of the free radicals.

The presence of phenethyl alcohol may be attributed to a carbonyl attack by aqueous electrons (that are more abundant) and its protonation.

According to the results mentioned above and literature,(8–10) a possible degradation pathway of acetophenone by pulsed corona discharges in water may be drawn as the following sequence:


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4. CONCLUSION The enhancement in acetophenone degradation in water by combination of pulsed corona discharges and addition of oxygen or ozone has been achieved. In the pulsed corona discharges in water, degradation of acetophenone is primarily attributed to pyrolysis, photolysis, and collisional dissociation. The degradation pathways have been proposed that account for the observed experimental data. The results obtained in pulsed corona discharges provide an approach to removal of toxic organic compounds in water. REFERENCES 1. N. J. Peill and M. R. Hoffmann, Environ. Sci. Technol. 30, 2806 (1996). 2. M. G. Nickelsen, W. J. Cooper, C. N. Kuruez, and T. D. Waite, Environ. Sci. Technol. 26, 144 (1991). 3. I. Hua and M. R. Hoffmann, Environ. Sci. Technol. 30, 864 (1996). 4. T. Yamamoto, K. Ramanathan, P. A. Lawless, D. S. Ensor, J. R. Newsome, N. Plakes, and G. H. Ramsey, IEEE Trans. Ind. Appl. 28(3), 528 (1992). 5. M. A. Malik and X. Z. Jiang, J. Environ. Sci. 10(3), 276 (1998). 6. R. G. Tonkyn, S. E. Barlow, and T. M. Orland, J. Appl. Phys. 80(9), 4877 (1996). 7. A. K. Sharma, B. R. Locke, P. Arce, and W. C. Finney, Hazardous Waste Hazardous Mater. 10, 209 (1993). 8. J. S. Clements, M. Sato, and R. H. Davis, IEEE Trans. Ind. Appl. IA–23, 224 (1987). 9. D. M. Willberg, P. S. Lang, R. H. Hochemer, A. Kratel, and M. R. Hoffmann, Environ. Sci. Technol. 30, 2526 (1996). 10. Y. M. Xu and C. H. Langford, J. Advan. Oxid. Technol. 2(3), 1 (1997). 11. F. D. Snell and L. S. Ettre, Encyclopedia of Industrial Chemical Analysis, Vol. 16, Wiley, New York (1972), p. 546. 12. A. A. Kulikovsky, IEEE Trans. Plasma Sci. 25, 439 (1997).